Polishing agent used for polishing semiconductor wafers and polishing method using the same

ABSTRACT

A semiconductor wafer polishing agent contains mainly a silica containing polishing agent and is added with a polyolefin type fine particle material. The novel semiconductor wafer polishing agent is capable of low brightness polishing to the back face of the wafer, sensor detection of the front and back faces of the wafer, and suppression of dust to be generated by chipping of the back face of the wafer, thereby to increase the yield of semiconductor devices. A polishing method using the polishing agent and a novel semiconductor wafer having a back face with an unconventional surface shape are also disclosed.

This is a divisional of application Ser. No. 08/670,258 filed Jun. 20,1996.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a polishing agent and a polishingmethod used for polishing semiconductor wafers, in particular,single-crystal silicon wafers (hereinafter may be referred to, forbrevity, as "wafer"). Further, this invention relates to a novelsemiconductor wafer having a back face with an unconventional surfaceshape.

2. Description of the Related Art

Generally, as shown in FIG. 10, the manufacturing method ofsemiconductor wafers includes a slicing process (A) to obtain wafers ofthin disc type by slicing a single crystal ingot formed by a pullingprocess using a crystal pulling machine; a chamfering process (B) tochamfer a peripheral edge portion of the wafer obtained through theslicing process (A) to prevent cracking or breakage of the wafer; alapping process (C) to flatten the surface of the chamfered wafer bylapping it; an etching process (D) to remove mechanical damage of the sochamfered and lapped wafer; a primary mirror polishing process (E1) topolish one side of the etched wafer to obtain a primary mirror surfaceof the wafer; a final mirror polishing process (G) to finally polish thesurface of the so primary mirror polished wafer to obtain a final mirrorsurface of the wafer; and a washing process (H) for washing the finallymirror polished wafer to remove the polishing agent or dust particlesfrom its surface.

As the etching process (D), there are two types of processes, that is,an acid etching process using an acid etching solution of a mixed acidor the like and an alkaline etching process using an alkaline etchingsolution of NaOH or the like. In the acid etching process, as shown inFIG. 11, a relatively high etching rate is obtained, and a surfaceroughness of an etched wafer is so fine that a cycle of the roughness isless than 10 μm and a P-V (Peak to Valley) value thereof is smaller than0.6 μm. On the contrary, in the alkaline etching process, as shown inFIG. 12, a surface roughness of an etched wafer is so large that a cycleof the roughness is in the range of 10 to 20 μm and a P-V value thereofsometimes exceeds 1.5 μm.

However, in the semiconductor wafer produced through the respectiveprocesses shown in FIG. 10, the following problem has been seen becausethe back face of the etched wafer is left as etched to the final stage.

After the both faces of the wafer are etched in the etching process,only the front face of the wafer is subjected to mirror polishing in thenext mirror polishing process. The polished front face of the wafer isnot chucked by as a vacuum chucking means and therefore offers noproblem. However, when the backs face of the etched wafer is chucked bysuch a chucking means, edged portions still remaining in the back facewith relatively large surface roughness are chipped or broken togenerate fine dust or a great number of fine particles, due to which theyield of semiconductor devices diminishes.

If both the front and back faces of the wafer are subjected to mirrorpolishing, there will be vanished relatively large surface roughness onthe back face. Therefore, the generation of fine dust or particlesdescribed above can be prevented so that the problem caused by such finedust or particles can also be solved.

However, according to the above both-face mirror polishing process, theback face also becomes a mirror surface. Thus, respective sensors ofprocessing machines can not distinguish the front face from the backface. Further, the so mirror polished wafer tends to slip out from aconveying line.

There have been no effective means capable of such low brightnesspolishing for semiconductor wafers that can satisfy the above facedetection and wafer conveyance.

SUMMARY OF THE INVENTION

With the foregoing problems in view, it is an object of the presentinvention to provide a novel semiconductor wafer polishing agent whichis capable of low brightness polishing to the back face of the wafer,sensor detection of the front and back faces of the wafer, andsuppression of fine dust or particles generated by chipping of the backface of the wafer, thereby to increase the yield of semiconductordevices.

Another object of the present invention is to provide a semiconductorwafer polishing method using such novel polishing agent for enablingnovel low brightness polishing of the semiconductor wafer.

Still another object of the present invention is to provide a novelsemiconductor wafer having a back face with an unconventional surfaceshape.

In one aspect, the present invention seeks to provide a semiconductorwafer polishing agent which contains mainly a silica containingpolishing agent and is added with a polyolefin type fine particlematerial.

The silica containing polishing agent includes a colloidal silicapolishing agent, and it is preferred to use a polyolefin aqueousdispersion as the polyolefin type fine particle material. The amount ofthe polyolefin type fine particle material is in the range of 0.01 to 1percent by weight, preferably 0.01 to 0.5 percent by weight, andoptimally 0.01 to 0.1 percent by weight relative to the total amount ofthe polishing agent.

The polyolefin type fine particle material or polyolefin aqueousdispersion includes the aqueous dispersions disclosed in Japanese PatentLaid-open Publication Nos. 4-46904, 4-88025 to 88026, 4-89830 to 89832and 4-218548 to 218549. Further, it is preferred to use CHEMIPEARL(trade name for a polyolefin aqueous dispersion manufactured by MitsuiPetrochemical Industries, Ltd.) which contains the above notedpolyolefin type fine particle material or polyolefin aqueous dispersion.

In another aspect, the present invention seeks to provide a method ofpolishing a semiconductor wafer characterized by low brightnesspolishing using the above described semiconductor wafer polishing agentfor polishing a semiconductor wafer.

In yet another aspect, the present invention seeks to provide asemiconductor wafer characterized by having a mirror surface as thefront face and a low brightness polished face, as the back face, formedby the above low brightness polishing method.

In yet another aspect, the present invention seeks to provide asemiconductor wafer characterized in that the front face is a mirrorsurface and the back face has many semi-spherical small projections.Preferably, the height of the projections is in the range of 0.05 to0.5μm, and the diameter thereof is in the range of 50 to 500 μm.

The most important feature of the present invention is that lowbrightness polishing can be achieved by forming the semi-spherical smallprojections having a height of 50 to 500 μm and a diameter of 0.05 to0.5 μm in the back face of a semiconductor wafer which is polished usinga silica containing polishing agent added with a polyolefin type fineparticle material. Accordingly, by mirror polishing the front face ofthe wafer and low brightness polishing the back face of the wafer,difference in brightness occurs between the front and back faces.Therefore, sensor detection of the front and back faces becomespossible. The term "brightness" denotes the percentage of the testedface reflectance relative to 100 of the perfect mirror surfacereflectance.

Further, there can be suppressed the generation of dust from the backface of the wafer which is subjected to the low brightness polishingusing the above silica containing polishing agent added with apolyolefin type fine particle material. For example, when the lowbrightness polished back face of the wafer is chucked in aphotolithography process of a device manufacturing line, the generationof dust by chipping can be suppressed and thereby the yield ofsemiconductor devices can be increased.

In addition, the use of the semiconductor wafer polishing agent of thepresent invention makes it possible to obtain a semiconductor waferhaving a novel back face shape.

The above and other objects, features and advantages of the presentinvention will become manifest to those versed in the art upon makingreference to the detailed description and the accompanying sheets ofdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photomicrograph of a wafer surface processed by lowbrightness polishing according to Example 1;

FIG. 2 is a photomicrograph of another surface portion of the waferprocessed by low brightness polishing according to Example 1 and a graphshowing the flatness of the surface portion;

FIG. 3 is a graph showing results of the measurement on the brightnessof wafer surfaces processed according to Example 1 and ComparativeExamples 1 to b 3;

FIG. 4 is a graph showing results of the evaluation on the generation ofdust of the wafer surfaces processed according to Example 1 andComparative Examples 1 to 3;

FIG. 5 is a diagram showing a method of evaluating the generation ofdust of the wafer surfaces processed according to Example 1 andComparative Examples 1 to 3;

FIG. 6 is a photomicrograph of a wafer surface processed by mirrorpolishing according to Comparative Example 1;

FIG. 7 is a photomicrograph of a wafer surface subjected to acid etchingaccording to Comparative Example 2;

FIG. 8 is a side elevation showing a polishing machine used in theExample and Comparative Examples;

FIG. 9 is a graph of the relationship between the amount of thepolyolefin type fine particle material and the polishing rate accordingto Example 1;

FIG. 10 is a flow chart showing one example of conventionalmanufacturing methods of semiconductor wafers;

FIG. 11 is a graph showing distribution of the surface roughness of awafer subjected to acid etching; and

FIG. 12 is a graph showing distribution of the surface roughness of awafer subjected to alkaline etching.

DETAILED DESCRIPTION

The present invention will be described below in greater detail by wayof the following examples which should be construed as illustrativerather than restrictive.

FIG. 8 shows an apparatus 10 for polishing a single crystal siliconwafer W, to carry out polishing processes in Experiment 1, Example 1 andComparative Examples 1-3.

In FIG. 8, the apparatus 10 comprises a rotary table assembly 12, arotary wafer carrier 13, and a polishing agent supplying member 14. Therotary table assembly 12 comprises a rotary table 15 and a polishing pad16 adhered on the upper surface of the rotary table 15. The rotary table15 can rotate on a shaft 17 at a predetermined rotation speed by adriving device such as a motor. The rotary wafer carrier 13 is forholding with a vacuum chucking means or the like means to carry thewafer W on the polishing pad 16 of the rotary table assembly 12 so thatthe surface of the wafer W faces to the polishing pad 16. The wafercarrier 13 can rotate on a shaft 18 at a predetermined rotation speedand horizontally move on the polishing pad 16 by an appropriate drivingdevice such as a motor. During operation of the apparatus 10, the waferW held by the wafer carrier 13 is in contact with the polishing pad 16and proper polishing loads are applied to the wafer W in a downwarddirection through the shaft 18 and the wafer carrier 13. The polishingagent supplying member 14 is for supplying a polishing agent 19 on thepolishing pad 16 to supply it between the wafer W and the polishing pad16.

In the following Experiment, Example and Comparative Examples, the backface of the wafer was first polished with the polishing apparatus 10under conditions specified therein, and the front face of the wafer wasthen subjected to mirror polishing after turning over of the wafer.

Experiment 1

Condition

Sample wafers: Czochralski-grown p-type, <100>-oriented,150-mm-diameter, single crystal silicon wafer

Polishing pad: Nonwoven fabric (velour-type), hardness=80 (AskerC-scale)

Polishing agent: 10.0 vol % of AJ-1325 (SiO₂ 2 wt %, pH 11, trade namefor a colloidal silica polishing agent manufactured by Nissan ChemicalIndustries, Ltd.), a polyolefin type fine particle material CHEMIPEARLS650 (tradename for a polyolefin aqueous dispersion manufactured byMitsui Petrochemical Industries, Ltd.)! and pure water (the rest)

Polishing load: 400 g/cm²

Polishing time: 10 min.

Under the condition specified above, the amount (wt %) of the polyolefintype fine particle material was changed with 0.025, 0.1, 0.45 and 1.0.Also, the amount of pure water was changed for the total amount of thepolishing agent so as to become 100 vol %. Using the polishing apparatusshown in FIG. 8, the back face of each sample wafer (two sheets for eachtest) was polished to measure the polishing rate in the polishingprocess. The results of the measurement are shown in FIG. 9.

As is apparent from the results of FIG. 9, against the level that thepolyolefin type fine particle material was not added, we found that thedegradation of polishing rate was seen little when the amount of thematerial was added in the range of 0.01 to 0.1 wt %, and that thepolishing process could be performed without serious degradation of thepolishing rate if the material was added in the range of 0.1 to 1%.

EXAMPLE 1

In the condition of Experiment 1, 0.025 wt % of CHEMIPEARL S650(tradename for a polyolefin aqueous dispersion, manufactured by MitsuiPetrochemical Industries, Ltd.) was added while keeping the otherconditions unchanged, and the back face of each sample wafer was thensubjected to polishing. Thereafter, the sample wafer was turned over tomake the front face of the wafer be subjected to mirror polishing usingthe same polishing apparatus. FIG. 1 is a photomicrograph of the backsface of the sample wafer processed in Example 1. FIG. 2 is aphotomicrograph of another back face portion of the same sample wafertogether with a graph showing the undulations in the back face measuredby a surface roughness tester.

Further, the brightness of the back face of the sample wafer wasmeasured. The results of the measurement are shown in FIG. 3. As isapparent from FIGS. 1 and 2, semi-spherical small projections having adiameter of 50 to 500 μm and a height of 0.05 to 0.5 μm were formed.Also, as is seen from FIG. 3, the brightness was 95% which led to thefact that the low brightness polishing could be achieved.

As shown in FIG. 5, a surface We for evaluation of the sample wafer Wwas pressed with load of 1 kg/cm² against a cleaned mirror surface Wm ofa mirror surface wafer W1. After the pressing step, the number of dustparticles (the number of particles whose size was larger than 0.1 μm) onthe cleaned mirror surface Wm transferred from the evaluation surface Weof the sample wafer W onto the cleaned mirror surface Wm of the mirrorsurface wafer W1 was counted using a particle counter. The results ofthe counting are shown in FIG. 4 in which the dust generation for thelow brightness polished surface We of the sample wafer W was evaluated.

As is clearly seen from FIG. 4, the number of the particles on the lowbrightness polished surface We was about 300. The result was near thenumber (200) of particle; on the mirror polished surface describedbelow. Thus, we could found that the dust generation in this Example wasvery low.

Comparative Example 1

In the polishing condition of Experiment 1, 10 vol % of AJ-1325 (SiO₂ 2wt %, pH 11, trade name for a colloidal silica polishing agentmanufactured by Nissan Chemical Industries, Ltd.) and 90 vol % of purewater was used as the polishing agent while keeping the other conditionunchanged, and both the front and back faces of each sample wafer werethen subjected to mirror polishing.

FIG. 6 is a photomicrograph of the surface of the mirror polished wafer.The brightness was measured in the same manner as in Example 1, and theresults of the measurement are shown in FIG. 3. Also, the evaluation onthe dust generation was conducted in the same manner as in Example 1,and the results thereof are shown in FIG. 4. As is clearly seen fromFIG. 6, undulations of large surface roughness on the mirror polishedsurface were not seen, and the brightness thereof was 100% as shown inFIG. 3. The number of particles counted to evaluate the dust generationwas about 200 which was extremely low.

Comparative Example 2

Using the same sample wafer as used in Experiment 1 which having beenprocessed up to the etching process where acid etching was conducted,only the front face of the wafer was subjected to mirror polishing inthe same condition as that in Experiment 1. FIG. 7 is a photomicrographof the back face, that is, the acid etched surface of the sample wafer.As is apparent from FIG. 7, undulations of fine roughness were formed onthe back face of the wafer, a cycle of which was less than 10 μm and aP-V (Peak to Valley) value of which was smaller than 0.6 μm. Also, thebrightness on the acid etched surface, that is, the back face of thesample wafer was measured, and the results of the measurement are shownin FIG. 3. The dust generation for the acid etched surface or the backface was then evaluated in the same manner as in Example 1, and theresults thereof are shown in FIG. 4. The brightness of the acid etchedsurface was 60% which was lower than that in Example 1. However, thenumber of particles counted to evaluate the dust generation was about700 which was quite higher than that obtained in Example 1.

Comparative Example 3

Using the same sample wafer as used in Experiment 1 which having beenprocessed up to the etching process where alkaline etching wasconducted, only the front face of the wafer was subjected to mirrorpolishing in the same condition as that in Experiment 1. The brightnesson the back face, that is, the alkaline etched surface of the samplewafer was then measured, and the results of the measurement are shown inFIG. 3. Thereafter, the dust generation for the alkaline etched surfaceor the back face was evaluated in the same manner as in Example 1, andthe results thereof are shown in FIG. 4. The brightness of the alkalineetched surface was 30% which was further lower than that in ComparativeExample 2. However, the number of particles counted to evaluate the dustgeneration was about 1500 which was quite higher than that obtained inComparative Example 2.

The same effect has been confirmed by experiment, even when n-typewafers have been used in place of the p-type wafers used in the examplesdescribed above.

Also, in the above Example, the polyolefin type fine particle materialwas added in the silica containing polishing agent. However, we havealso confirmed that the same effect can be obtained even in case ofadding agents, for example, ethylenediamine and the like which areusually added to the silica containing polishing agent.

As stated above, smooth semi-spherical projections can be formed on theback face of the wafer by polishing the face using the semiconductorwafer polishing agent of the present invention, thereby to lower thebrightness of the back face of the wafer. Thus, sensor detection of thefront and back faces of the wafer becomes possible, and the yield ofsemiconductor devices can be increased by suppressing the generation ofdust to be caused by chipping on the back face of the wafer. Inaddition, it becomes possible to obtain a semiconductor wafer having anovel back face shape by using the semiconductor wafer polishing agentof the present invention.

Obviously, various minor changes and modifications of the presentinvention are possible in the light of the above teaching. It istherefore to be understood that within the scope of the appended claimsthe invention may be practiced otherwise than as specifically described.

What is claimed is:
 1. A method of polishing a semiconductor wafer,comprising polishing which uses a semiconductor wafer polishing agentcomprising a silica containing polishing agent as a main component and apolyolefin particle material as an additive for polishing asemiconductor wafer.
 2. A method of polishing a semiconductor waferaccording to claim 1, wherein said silica containing polishing agent isa colloidal silica polishing agent.
 3. A method of polishing asemiconductor wafer according to claim 1, wherein said polyolefinparticle material is a polyolefin aqueous dispersion.
 4. A method ofpolishing a semiconductor wafer according to claim 2, wherein saidpolyolefin particle material is a polyolefin aqueous dispersion.
 5. Amethod is polishing a semiconductor wafer according to claim 1, whereinthe amount of said polyolefin particle material is in the range of 0.01to 1 percent by weight.
 6. A method of polishing a semiconductor waferaccording to claim 2, wherein the amount of said polyolefin particlematerial is in the range of 0.01 to 1 percent by weight.
 7. A method ofpolishing a semiconductor wafer according to claim 3, wherein the amountof said polyolefin particle material is in the range of 0.01 to 1percent by weight.
 8. A method of polishing a semiconductor waferaccording to claim 4, wherein the amount of said polyolefin particlematerial is in the range of 0.01 to 1 percent by weight.